REVIEW OF THE PRIOR ART
Many ingenious and sophisticated proposals have been made in the field of ocean engineering calling for the use of large diameter vertical ducts of great length. These ducts extend from at or near the ocean surface to lower ends unconnected to the ocean floor. These proposals include concepts for ocean thermal energy conversion and for mariculture. The ducts involved in these proposals are sometimes referred to as riser pipes.
The concepts for ocean thermal energy conversion propose to use the difference in thermal energy levels between warm surface water and colder deepwater to generate electricity, for example. The available energy level difference is low and so these proposals rely on the use of very large quantities of warm and cold water. These proposals call for the necessary large volumes of deep cold water to be brought to the water surface through very large vertical ducts of great length. The mariculture proposals typically call for large quantities of cold nutrient-rich deep water to be brought to the warmer surface portions of the ocean to provide food for the growth of ocean plants and animals. For example, one proposal calls for kelp to be grown on frames located about sixty feet or so below the ocean surface and to be nourished by nutrient-rich water brought up to the vicinity of the frames from 1500 feet or more below the ocean surface.
These proposals and others similar to them have the common feature of requiring the use of very long relatively large diameter vertical ducts through which water from deep in the ocean may flow upwardly, i.e., upwell. The lower ends of these ducts may be located a substantial distance above the ocean floor and may be free, i.e., not anchored or otherwise held in fixed position by a connection to the ocean floor.
Any structure which extends vertically for any significant distance in the ocean will encounter at least one ocean current. Currents imposed drag forces upon such structures. The larger the structure, the greater its profile (effective area) presented to the current, and therefore the greater the drag forces which a given current will impose upon the structure. The drag forces applied to upwelling ducts produce deflection of the ducts. If the lower end of the duct is free, such deflections result in movement of the lower end of the duct away from its desired position. Often the duct will oscillate or whip about in the ocean in reaction to the current drag forces applied to the duct. Such motions of the duct may lead to damage of the duct or will reduce its useful life.
The problem of deflection of a long vertically disposed duct by ocean currents could be dealt with by stiffening the duct. Stiffening may be accomplished by making the duct of a rigid material, such as a metal. This solution, however, has two drawbacks, namely, the duct becomes heavy and requires larger and larger support structures, and the stresses introduced in the duct by bending moments generated by current drag forces may reach critical levels. The problem of high stress levels can be handled by making the duct walls thicker, but this in turn adds to the overall weight of the duct. Alternatively, the difficulty presented by critical stress levels may be resolved by making the duct flexible. A flexible duct, however, will deflect significantly. A duct 2000 feet long for collecting water at 2000-foot depths may be so significantly deflected by ocean currents as to have its lower end located only about a thousand feet below the water surface.
It is therefore seen that the need exists for techniques and arrangements for stabilizing the position of the free lower end of a suspended elongate duct in an ocean.